1. Field of the Invention
The present invention relates to a polishing agent and a polishing method used for polishing semiconductor wafers, in particular, single-crystal silicon wafers (hereinafter may be referred to, for brevity, as "wafer"). Further, this invention relates to a novel semiconductor wafer having a back face with an unconventional surface shape.
2. Description of the Related Art
Generally, as shown in FIG. 10, the manufacturing method of semiconductor wafers includes a slicing process (A) to obtain wafers of thin disc type by slicing a single crystal ingot formed by a pulling process using a crystal pulling machine; a chamfering process (B) to chamfer a peripheral edge portion of the wafer obtained through the slicing process (A) to prevent cracking or breakage of the wafer; a lapping process (C) to flatten the surface of the chamfered wafer by lapping it; an etching process (D) to remove mechanical damage of the so chamfered and lapped wafer; a primary mirror polishing process (E1) to polish one side of the etched wafer to obtain a primary mirror surface of the wafer; a final mirror polishing process (G) to finally polish the surface of the so primary mirror polished wafer to obtain a final mirror surface of the wafer; and a washing process (H) for washing the finally mirror polished wafer to remove the polishing agent or dust particles from its surface.
As the etching process (D), there are two types of processes, that is, an acid etching process using an acid etching solution of a mixed acid or the like and an alkaline etching process using an alkaline etching solution of NaOH or the like. In the acid etching process, as shown in FIG. 11, a relatively high etching rate is obtained, and a surface roughness of an etched wafer is so fine that a cycle of the roughness is less than 10 .mu.m and a P-V (Peak to Valley) value thereof is smaller than 0.6 .mu.m. On the contrary, in the alkaline etching process, as shown in FIG. 12, a surface roughness of an etched wafer is so large that a cycle of the roughness is in the range of 10 to 20 .mu.m and a P-V value thereof sometimes exceeds 1.5 .mu.m.
However, in the semiconductor wafer produced through the respective processes shown in FIG. 10, the following problem has been seen because the back face of the etched wafer is left as etched to the final stage.
After the both faces of the wafer are etched in the etching process, only the front face of the wafer is subjected to mirror polishing in the next mirror polishing process. The polished front face of the wafer is not chucked as a vaccum chucking means and therefore offers no problem. However, when the back face of the etched wafer is chucked by such a chucking means, edged portions still remaining in the back face with relatively large surface roughness are chipped or broken to generate fine dust or a great number of fine particles, due to which the yield of semiconductor devices diminishes.
If both the front and back faces of the wafer are subjected to mirror polishing, there will be vanished relatively large surface roughness on the back face. Therefore, the generation of fine dust or particles described above can be prevented so that the problem caused by such fine dust or particles can also be solved.
However, according to the above both-face mirror polishing process, the back face also becomes a mirror surface. Thus, respective sensors of processing machines can not distinguish the front face from the back face. Further, the so mirror polished wafer tends to slip out from a conveying line.
There have been no effective means capable of such low brightness polishing for semiconductor wafers that can satisfy the above face detection and wafer conveyance.